U.S. patent number 5,278,588 [Application Number 07/702,582] was granted by the patent office on 1994-01-11 for electrographic printing device.
This patent grant is currently assigned to Delphax Systems. Invention is credited to Igor Kubelik.
United States Patent |
5,278,588 |
Kubelik |
January 11, 1994 |
Electrographic printing device
Abstract
An electrographic printhead is operated to focus charged
particles on a print member. In one embodiment successive
electrodes are maintained at potentials that define increasing
electric field strengths in the successive regions along the
particle trajectory. In another embodiment, a thick electrode face
defines equipotential lines that shape a sharply focusing
electrostatic field that penetrates the projection apertures of the
printhead. Improved focusing diminishes charge spreading effects,
and allows effective positioning of the printhead at greater
spacings to reduce the risk of arcing without loss of print
resolution.
Inventors: |
Kubelik; Igor (Mississauga,
CA) |
Assignee: |
Delphax Systems (Canton,
MA)
|
Family
ID: |
24821818 |
Appl.
No.: |
07/702,582 |
Filed: |
May 17, 1991 |
Current U.S.
Class: |
347/127 |
Current CPC
Class: |
B41J
2/415 (20130101) |
Current International
Class: |
B41J
2/415 (20060101); B41J 2/41 (20060101); G01D
015/06 () |
Field of
Search: |
;346/159 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller, Jr.; George H.
Attorney, Agent or Firm: Lahive & Cockfield
Claims
What is claimed is:
1. A printhead for depositing a pointwise pattern of charge on an
imaging member, such printhead comprising
generation means including an array of generation electrodes
actuable to generate charged particles in a localized region,
extraction means for extracting charged particles from the region
and including at least one layer of apertured electrode elements
wherein apertures define the positions of latent charge image dots,
and
electrical means for establishing a potential difference between
electrode elements and the imaging member defining a focusing field
for focusing charged particles as they are extracted through the
apertures and directed toward the imaging member.
2. The printhead of claim 1, wherein the extraction means includes
first and second apertured electrode elements spaced apart along a
direction of travel of the charge particles, and wherein the
electrical means applies potentials to the electrode elements such
that a beam of charged particles is focused in passing through an
aperture of the first electrode element, and is focused in passing
through an aperture of the second electrode element, thereby
narrowing the beam.
3. The printhead of claim 2, wherein the electrical means applies
plural different potentials to the first electrode, the second
electrode and the imaging member to maintain a greater acceleration
field strength between the second electrode and imaging member,
than between the first electrode and the second electrode.
4. The printhead of claim 3, wherein the electrical means applies a
potential difference greater than 500 volts between the imaging
member and the second electrode.
5. The printhead of claim 1, wherein a layer of apertured electrode
elements nearest the imaging member includes a thick conductive
element having a face defining an extraction aperture that is
beveled outwardly toward the imaging member to define equipotential
lines of a focusing extraction field that extends into the
aperture.
6. An improved printing system of the type wherein an
electrographic printhead is positioned opposite an imaging surface
to deposit a pointwise pattern of charge dots on the imaging
surface for development into a visible image, wherein the
improvement resides in the printhead comprising
generation means including an array of generation electrodes
actuable to generate charged particles in a localized region,
extraction means for extracting charged particles from the region
and including at least one layer of apertured electrode elements
wherein apertures define the positions of latent charge image dots,
and
electrical means for establishing a potential difference between
electrode elements and the imaging member defining a focusing field
for focusing charged particles as they are extracted through the
apertures and directed toward the imaging member whereby dot
blooming of deposited charge is diminished.
7. The improved printing system of claim 6, wherein the extraction
means includes first and second apertured electrode elements spaced
apart along a direction of travel of the charge particles, and
wherein the electrical means applies potentials to the electrode
elements such that a beam of charged particles is focused in
passing through an aperture of the first electrode element, and is
focused in passing through an aperture of the second electrode
element, thereby narrowing the beam.
8. The improved printing system of claim 7, wherein the electrical
means applies plural different potentials to the first electrode,
the second electrode and the imaging member to maintain a greater
acceleration field strength between the second electrode and
imaging member, than between the first electrode and the second
electrode.
9. The improved printing system of claim 8, wherein the electrical
means applies a potential difference greater than 500 volts between
the imaging member and the second electrode.
10. The improved printing system of claim 6, wherein a layer of
apertured electrode elements nearest the imaging member includes a
thick conductive element having a face defining an extraction
aperture that is beveled outwardly toward the imaging member to
define equipotential lines of a focusing extraction field that
extends into the aperture.
11. The improved printing system of claim 6, wherein the printhead
is positioned at least 0.4 mm from the imaging member to deposit a
non-diverging beam of charged particles while decreasing the
likelihood of arcing.
Description
BACKGROUND OF THE INVENTION
The present invention relates to electrographic printers of the
type wherein a printhead generates charge carriers and directs them
at a recording or imaging member to form a desired image by the
selective activation of electrodes. It is particularly directed to
such printers wherein one set of electrodes is activated as a
source of charge carriers, e.g., ions or electrons, and a second
set of electrodes is activated to extract and accelerate the charge
carriers toward the latent imaging member.
Printheads of this type are described in U.S. Pat. Nos. 4,160,257,
4,628,227, 4,992,807, and others. In the printheads described more
particularly in the aforesaid patents, a set of electrodes are
activated with an RF frequency signal of up to several thousand
volts amplitude to create a localized corona or glow discharge
region. Lesser control voltages are applied to one or more control
electrodes located at or near the discharge region to gate positive
or negative charge carriers from the region, and the printhead is
biased with respect to a dielectric member to maintain an
accelerating field therebetween, so that the charge carriers are
drawn from the printhead and deposited as charge dots constituting
a latent image on the dielectric imaging member as it moves past
the printhead.
In printing devices using this type of printhead, the RF-driven
corona generation lines extend along the width of the printhead,
spanning many of the control electrodes, which cross them at an
angle. One commercial embodiment, by way of example, has twenty
parallel RF lines, which are crossed by one hundred twenty eight
oblique control electrodes, known as finger electrodes. During the
time when one RF line is activated by a burst of approximately five
to ten cycles of a one to three MHz drive signal with a peak to
peak amplitude of approximately 2700 volts, those finger electrodes
which cross the RF line at the desired dot locations are activated
to deposit charge dots.
In the conventional drive circuitry for such systems, the RF drive
lines are actuated in a fixed sequence independent of the image
being printed, while during any given RF line actuation, the number
of finger electrodes which are actuated varies in accordance with
the required number and location of dots for the pattern being
printed. After a slight delay for the RF voltage to ramp up, the
designated finger electrodes are turned on to cause charge carriers
to pass from the printhead and accelerate toward the drum, belt or
other latent imaging member. Specifically, during their "OFF"
cycle, each finger is back biased by several hundred volts with
respect to the screen voltage. During its "ON" cycle, the finger
voltage is switched to approximately the same potential as the
screen, so charge carriers of one polarity reaching the screen
aperture are drawn to the imaging member.
In the original printers of this type, the finger electrodes were
switched on for a fixed interval substantially co-extensive with
the RF corona generation burst. Such operation produces a fixed
amount of charge per actuation. More recently, in U.S. Pat. No.
4,841,313 or 60 Nathan K. Weiner, constructions with a finger pulse
of varying duration have been proposed. This operation varies the
amount of charge deposited at each dot. In U.S. Pat. No. 4,992,807
of Christopher W. Thomson, other control regimens involving varying
voltage levels or potential differences in the front electrodes or
electrode structures have been described.
Printheads of the aforesaid type are generally operated at a
relatively small gap of about 0.25 mm from the image-receiving belt
or drum surface, and are biased, with respect to the imaging
member, to maintain a relatively high electrostatic acceleration
field of 2-3 KV/mm in the gap. The size of the charged particle
beams generated by the printhead decreases with higher acceleration
field. Considerations of assuring a dependable firing threshold
while not risking the occurrence of arcing generally prevent the
use of extreme values for either printhead gap or acceleration
field operating parameters, and dictate bias voltages and gap
spacings in the range indicated above.
The operation of such closely-spaced imaging member and printhead
electrode arrays at voltages effective to provide small beam
dispersion requires voltages as high as fifty percent or more of
the spark breakdown voltage, and may lead to erratic arcing or
irregular toning of the latent image, so various attempts have been
made in the past to reduce the potential difference across the
printhead-to-drum gap. In U.S. Pat. No. 4,658,275 a construction is
proposed that places a second screen electrode between the
printhead and an imaging belt, with the second screen electrode and
a conductor on the opposite side of the imaging belt maintained at
the same potential to eliminate any electric field in the gap and
prevent extraneous charging or toning of the belt. Others in the
industry have proposed additional electrodes located closer to a
drum with a grounded core, and maintained at potentials closer to
ground to permit reduction of the printhead-to-drum gap, and hence
limit the beam divergence.
Generally, charge-deposition printheads deposit a quantity of
charge for each print dot in an amount that is sufficient to
attract and hold toner onto the imaging member. For one typical
dry-toned embodiment, the latent image surface potential required
for toning may be between fifty and several hundred volts in
charged areas. With such a significant potential, as charge is
deposited on the imaging member, the latent image electric field
builds up to such magnitude that the projected charge particle beam
becomes increasingly divergent, so that the latent image charge dot
spreads out. This charge spreading effect can result in the
deflection of a substantial portion of the charge of one dot into
an annulus outside of the intended dot area, "spreading" the dot
dimension by several mils. When the printhead dimensions and
operating parameters of a system have been optimized to print
images with a resolution of several hundred dots per inch or more,
the charge spreading effect can degrade print quality, resulting in
loss of print density, loss of print detail, and blurring of color
separation in multiply-toned prints.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to diminish charge
spreading effects.
It is another object of the invention to provide a well focused
charged particle beam array while reducing arcing and unwanted
discharge between a printhead and an imaging member.
These and other objects of the invention are attained in a
printhead structure wherein a first array of electrodes generates
charged particles, and a second array, preferably comprising a
first screen electrode surface and a second screen electrode
surface energized with different potentials, forms successively
greater acceleration fields between the first array and an imaging
member. In the preferred embodiment, the distance d.sub.1 between
the first screen electrode and the second screen electrode is
advantageously substantially less than the distance d.sub.2 between
the second screen electrode and the imaging member, and the second
screen is maintained at a relatively high potential difference with
respect to the imaging member. In particular, for depositing a
maximum latent image potential V.sub.max, the second screen/drum
voltage difference V.sub.s and the voltage difference V.sub.f
between the first and the second screens satisfy ##EQU1## In
another embodiment the second array is formed of a single thick
conductor, having a transversely-oriented surface defining
equipotential lines of a non-linear aperture-penetrating focusing
field. In this embodiment, the aperture is preferably beveled
outwardly toward the imaging member.
In preferred embodiments of the two screen printhead, the screens
are separated by a distance that is substantially less than the
printhead gap, and preferably the printhead array is positioned at
least 0.2 mm from the imaging member.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will be understood from
the description herein, taken together with the drawings
wherein:
FIGS. 1, 2 and 2A illustrate prior art printer or printhead
constructions and variations thereof;
FIGS. 3A and 3B illustrate charge spreading and field effects in a
prior art printer;
FIG. 4 illustrates in cross-section a printhead in accordance with
the present invention;
FIGS. 5A to 5C illustrate field lines and charge particle
trajections of the printhead of FIG. 4 with different applied
voltages; and
FIGS. 6, 7 and 8 illustrate other embodiments of printheads in
accordance with the present invention.
DETAILED DESCRIPTION
FIG. 1 illustrates a printhead 2 and a drum imaging member 40 of a
prior art system described above, wherein charged particles 41 are
directed from apertures 11 to deposit charge dots constituting a
latent charge image on the surface of the member 40, which may be a
drum, belt, sheet recording member, or the like. The gap "g" is
greatly exaggerated for clarity of illustration.
In this construction a first array of electrodes constituted by a
set of longitudinal drive electrodes 8 and a set of transverse
"finger" electrodes 12 are actuated by driver units 20, 30 to
develop localized pools of charged particles in regions 12c
adjacent to edges 12a, 12b of the finger electrodes 12. A large
potential difference exists between the finger electrodes 12 and a
ground plane (not shown) just below the surface of member 40, and a
screen electrode 10 positioned between the first array and member
40 shields the charge generating structure from the high field in
the printhead-drum gap. Screen electrode 10 is shown as a
continuous conductive sheet having apertures in registry over the
electrode crossing points of the driver/finger array but the screen
may be implemented as a plurality of separate screen electrodes,
each over one or more, e.g., a row or column of, apertures. Also,
the screen apertures may be slots that span a plurality of
different charge generating loci. In lieu of a single screen
electrode over each crossing point, two or more layers of electrode
structures may be provided to shield the first electrode array
while extracting or accelerating charge carriers toward the
drum.
FIG. 2 illustrates in cross-section one electrode set of an
embodiment of a prior art printhead 50 employing two screen
electrodes 51, 53 in its charge extraction system, as appears in
U.S. Pat. No. 4,658,275. A plurality of insulating layers 54
separate the various electrodes, and an RF driver/finger electrode
array, numbered identically to the corresponding structure in FIG.
1, provides a source of charged particles. That system applies
charge to the back side of a dielectric belt 42, and a conductive
toner roll R applies toner particles T to adhere to the other side
of the belt. In this construction, a common potential is applied
between the toner reservoir and the front electrode, so no
acceleration field f.sub.2 exists in the printhead-to-belt gap. In
the more commonly available commercial embodiments of "ionic" or
electrographic printing wherein a drum or belt is charged and toner
is applied from the same side in a later step, the conductive toner
roll would correspond to a conductive backplane commonly provided
as a sublayer of the imaging belt or drum. Such a hypothetical
modification of the prior art is illustrated in FIG. 2A.
Turning now to FIGS. 3A, 3B there is illustrated in a schematic
manner the equipotential electric field lines "ef.sub.i " of a
conventional single-screen printhead at one charge projecting
electrode set during different phases of a charge-depositing
operation. By way of scale, the gap between screen electrode 10 and
imaging member 40 is typically about 0.2 mm, and the total
potential difference 400-700 volts. The equipotentials are
identified by sequential numbers, ef.sub.l, ...ef.sub.n to indicate
their relative positions near to the screen (low subscripts) or
near to the imaging member (subscripts close to n.) As illustrated,
the field shape near the screen has a moderately convergent or
focusing effect on the beam, but quickly flattens out so that once
the charged particles have left the printhead they are accelerated
along, but not appreciably diverted from their parting trajectory.
This model applies to operation of the screen when no charge has
yet been deposited on the imaging member and the bias with respect
to the conductive backplane 42 presents a substantially uniform
accelerating field.
As a charge dot is deposited, however, the surface potential on the
imaging member rises by up to several hundred volts. This not only
reduces the accelerating potential in the gap, but produces a
locally non-uniform electric field. As shown in FIG. 3B, the
equipotential lines in this later stage are therefore bent as they
approach the latent image dot, with those lines closer to the
imaging surface producing a diverging effect on the trajectories of
charge carriers 41. The particle bundle therefore blooms outwardly,
broadening the charge dot. The exact beam trace size depends in
part on how narrow the beam is as it leaves the printhead
aperture.
Applicant has calculated that the foregoing dot size can be
substantially reduced by providing an electrode geometry and gap
field such that the original charge carrier beam is highly focused.
This is achieved by novel electrode geometries together with the
application of focusing potentials as described further below.
One such printhead electrode structure 100 is illustrated in FIG.
4. In this embodiment a rear charge generation structure including
RF and finger electrodes 8, 12 separated by an insulating film 111
generates particles that are accelerated out by an extraction
assembly 101, 102, 103 spaced apart by a spacer layer 112 that
defines a glow chamber 107. Insulating RF electrode coating 110 is
shown for completeness, but is not material to the inventive aspect
of this printhead.
Between driver 8 and first screen electrode 101 the assembly is
substantially identical to the device of FIGS. 3A, 3B, and the
electric field lines in operation are also substantially as shown
therein. Structure 100 contains, in addition a second screen
electrode 102, which in the preferred embodiment is both closely
spaced to the screen 101, and preferably also has wider apertures
than screen 101. Most basic to the invention is the maintenance of
a higher acceleration field between screen 102 and the dielectric
member 40, than the field existing between the two screens, from
which the desirability of these other two properties follows.
Specifically, the higher acceleration field is maintained so that
at the aperture of screen 102 a focusing electric field, indicated
by equipotential line FF, exists.
It will be appreciated that the screen apertures are quite small,
e.g, 0.1 to 0.2 millimeters diameter, so that the field produced by
the infinite plate 42 penetrates at most a small distance into the
apertures. By making the apertures of screen 102 larger than (e.g.,
about 1.1 to 2 times as large as) those of screen 101 the strong
acceleration external field presents a focusing contour across the
whole aperture and extending deeply toward electrode 101.
It will further be appreciated that the efficiency of extraction of
charge carriers from the glow chamber 107 depends on the presence
of a strong accelerating field to capture the particles at the
inner aperture of screen 101. Desirably, the field gradient should
be in the range of 1-2,000 V/mm.
In order to obtain a suitable extraction field yet still be able to
establish a stronger field between elements 102 and 40, the inner
acceleration field is achieved by providing a thin spacer layer 103
between the two screens, and applying only a moderate potential
therebetween. For example, with a potential difference of fifty
volts and spacer layer 103 one or two mils thick an extraction
gradient of 1000 V/mm to 2000 V/mm is achieved. By positioning the
outer screen 102 the usual spacing of 0.2 millimeters from member
40 at a near normal bias of 500 volts a higher accelerating field
gradient of 2500 V/mm is achieved. Operated in this manner the
double screen configuration creates a strongly focusing
electrostatic lens and thus the beam is very narrowed. It is
understood that if the gap is doubled, thus decreasing the
acceleration gradient, the interscreen potential must be decreased
accordingly, or the thickness of spacer 103 increased, in order to
maintain the external field strength greater than the inter-screen
field strength so that both screens together perform a focusing
effect.
FIGS. 5A-5C illustrate the effects of the relative field strengths
of the interscreen and the screen-to-drum spaces, specifically
FIGS. 5A-5C illustrate the different focusing or diverging effects
obtained with different relative magnitudes of electric field
E.sub.1, E.sub.2, and E.sub.3 within the cavity 107, between the
screen electrodes, and in the printhead-to-drum gap, respectively.
A representative field line is drawn on each side of the transition
regions, together with an indication of whether the electrostatic
lensing effect is focusing (F) or diverging (D). In practice, the
cavity field is poorly understood owing to the intense corona
activity inside, and the rapidly changing RF oscillations. The
finger screen bias is therefore set in a conventional manner to
gate a desired amount of charge. As shown in the diagrams, however,
the relative strengths of E.sub.2 and E.sub.3 are important, with
the condition of E.sub.3 >E.sub.2 assuring a further focusing
effect.
It will be noted that the front screen electrode of these Figures
is shown as having an aperture size identical to that of the rear
screen electrode. This may help to form an effective intermediate
field with relatively low potential differences.
Further embodiments for achieving enhanced charge deposition are
shown in FIGS. 6-9.
As illustrated in FIG. 6, the front screen electrode is coated with
a film 120. This may be a vapor- or solvent-deposited coating, a
sputtered-on dielectric, or other coating. It may be a conductive
coating that is applied as a protective coat against arcing and
corrosive byproducts, but preferably is a thin dielectric coating
which is charged by the covered screen electrode to provide a
relatively smooth field around the aperture edges. The inner screen
remains uninsulated.
FIG. 7 shows another embodiment, wherein a single screen electrode
130 is employed. Screen 130 is a thick screen, such that its face
defining the extraction aperture provides a constant-potential
funnel for shaping the external gap field into a deeply penetrating
focusing field. That is, the aperture may be outwardly beveled
toward the imaging member. The thick electrode may be formed by
several layers 130a, 130b joined along surface 135. The bevel may
be fabricated by etching. FIG. 8 shows a related embodiment,
wherein a beveled opening is provided through electrically
separated screen electrodes.
In one representative prototype of a printhead constructed in
accordance with the invention, and having the structure illustrated
in FIG. 4, the opening in finger electrode 12 was six mils in
diameter and spacer 112 defined a cavity six mils in depth The
first screen 101 was formed of one mil thick stainless steel having
a 7.5 mil aperture, which was spaced two mils from the front screen
102. Screen 102 was formed of identical material and had apertures
of 9.5 mil diameter in registry with those of the underlying
structure. An interscreen potential difference of approximately
fifty volts, and a screen drum potential difference of
approximately six hundred fifty volts were applied, at a gap of 0.3
mm, providing successive accelerating field of E.sub.2 =1000 V/mm
and E.sub.3 =2200 V/mm, respectively.
The foregoing constructions illustrate the principles of the
invention, but are subject to variation and modification in
accordance with the numerous considerations involved in
implementing printers of different architectures. For example, one
of the two screen electrodes may be implemented as plurality of
separately energized strip electrodes and the voltage on each strip
may be varied to adjust the beam diameter to compensate for drum
curvature effects. Another construction contemplated by the
invention is to hold constant the high-voltage front screen to drum
potential, and vary the voltage of the inner screen to adjust the
amount of charge extracted from the printhead. This adjustment may
be used to control print density.
The invention being thus described, other variations and
modifications thereof will occur to those skilled in the art, and
all such variations and modifications are considered to lie within
the scope of the invention to which an exclusive right is claimed,
as defined in the claims appended hereto.
* * * * *